Ion – Induced Nucleation of Sulphuric Acid and Water

The classical ion-induced nucleation theory is applied to study the formation of new aerosol particles in the binary vapor mixture of sulfuric acid and water in the presence of pre-existing aerosol particles and ion source. The results of a case study for the defined steady state conditions (T = 273 K, RH = 50%) show that the total (free molecules + hydrates) number concentrations of sulfuric acid higher than 3·10 cm are needed to obtain ion-induced nucleation rates higher than 0.1 stable particles with the radius of ≈0.8 nm (mobilities ≈ 0.5 cm/(V·s)) per cms. At the acid concentrations higher than 3.3·10 cm the nucleation on ions is barrierless and the hydration of acid molecules in vapor has no effect on nucleation. In the interplay with aerosol scavenging and ion recombination processes, ion-induced nucleation starts to cut down the concentration of small ions at total sulfuric aid concentrations more than about 10 cm. For a typical tropospherical aerosol load and ion generation rate (10 ion pairs per cms) and at acid vapor concentrations (≈4·10 cm), where ion-induced and homogeneous nucleation rates are approximately equal (≈3 cms), ion-induced nucleation reduces small air ion concentration less than about 35%. INTRODUCTION In many places in the atmosphere the observed nucleation bursts of new aerosol particles are accompanied with the increase in sulfuric acid concentration, but usually the observed concentrations are too low for explaining the occurrence of these bursts with the homogeneous binary nucleation of sulfuric acid and water. One mechanism that lowers the concentration of vapors needed for nucleation is nucleation on ions. In this report, ion-induced nucleation is studied in the interplay with aerosol scavenging and ion recombination. BALANCE EQUATIONS OF ION-INDUCED NUCLEATION In the atmosphere, the time for a ion cluster to lose or gain an acid molecule or a hydrate (a cluster containing one acid molecule and one or more water molecules) is much longer than the time needed to reach the equilibrium state with respect to water vapor. Hence the distribution of clusters f(a,w) by the number of the molecules of water w and acid a in a cluster is proportional to an equilibrium distribution: f(a,w) = faN(a,w), where the coefficient of proportionality fa is a function of the number of acid molecules a. The clusters are able to grow or evaporate by adding or losing monomers and hydrates. We neglect the possible increase of condensation due to the interaction between ions and condensing dipolar vapor molecules. The small air ions (ions with mobilities larger and radiuses smaller than 0.5 cm/(V·s and 0.8 nm, respectively) are considered to interact with each other through the recombination and are scavenged by pre-existing aerosol particles. The influence of newly formed particles on the processes considered is ignored. On the assumption of charge symmetry the evolution of a size distribution of ion clusters is described by equations [Noppel, 1996]: a a a i a a a w f L D f N J J t f w a N π α 4 ) , ( 1 − − − = ∂ ∂ Σ + ∑ , (1) where Ja is the clusters' current or net transport of clusters from the size class of a-1 acid molecules to the size class of a molecules by the condensation-evaporation process, i.e., Ja=βa-1 fa-1 – aa fa, (2)